Potassium (K+) channels are ubiquitous proteins which are involved in the setting of the resting membrane potential as well as in the modulation of the electrical activity of cells. In excitable cells, K+ channels influence action potential waveforms, firing frequency, and neurotransmitter secretion (Rudy, B. (1988) Neuroscience, 25, 729-749; Hille, B. (1992) Ionic Channels of Excitable Membranes, 2nd Ed.). In non-excitable cells, they are involved in hormone secretion, cell volume regulation and potentially in cell proliferation and differentiation (Lewis et al. (1995) Annu. Rev. Immunol., 13, 623-653). Developments in electrophysiology have allowed the identification and the characterization of an astonishing variety of K+ channels that differ in their biophysical properties, pharmacology, regulation and tissue distribution (Rudy, B. (1988) Neuroscience, 25, 729-749; Hille, B. (1992) Ionic Channels of Excitable Membranes, 2nd Ed.). More recently, cloning efforts have shed considerable light on the mechanisms that determine this functional diversity. Furthermore, analyses of structure-function relationships have provided an important set of data concerning the molecular basis of the biophysical properties (selectivity, gating, assembly) and the pharmacological properties of cloned K+ channels.
Functional diversity of K+ channels arises mainly from the existence of a great number of genes coding for pore-forming subunits, as well as for other associated regulatory subunits. Two main structural families of pore-forming subunits have been identified. The first one consists of subunits with a conserved hydrophobic core containing six transmembrane domains (TMDs). These K+ channel α subunits participate in the formation of outward rectifier voltage-gated (Kv) and Ca2+-dependent K+ channels. The fourth TMD contains repeated positive charges involved in the voltage gating of these channels and hence in their outward rectification (Logothetis et al. (1992) Neuron, 8, 531-540; Bezanilla et al. (1994) Biophys. J. 66, 1011-1021).
The second family of pore-forming subunits have only two TMDs. They are essential subunits of inward-rectifying (IRK), G-protein-coupled (GIRK) and ATP-sensitive (KATP) K+ channels. The inward rectification results from a voltage-dependent block by cytoplasmic Mg2+ and polyamines (Matsuda, H. (1991) Annu. Rev. Physiol., 53, 289-298). A conserved domain, called the P domain, is present in all members of both families (Pongs, O. (1993) J. Membr. Biol., 136, 1-8; Heginbotham et al. (1994) Biophys. J. 66,1061-1067; Mackinnon, R. (1995) Neuron, 14, 889-892; Pascual et al., (1995) Neuron., and 14, 1055-1063). This domain is an essential element of the aqueous K+-selective pore. In both groups, the assembly of four subunits is necessary to form a functional K+ channel (Mackinnon, R. (1991) Nature, 350, 232-235; Yang et al., (1995) Neuron, 15, 1441-1447.
In both six TMD and two TMD pore-forming subunit families, different subunits coded by different genes can associate to form heterotetramers with new channel properties (Isacoff et al., (1990) Nature, 345, 530-534). A selective formation of heteropolymeric channels may allow each cell to develop the best K+ current repertoire suited to its function. Pore-forming α subunits of Kv channels are classified into different subfamilies according to their sequence similarity (Chandy et al. (1993) Trends Pharmacol. Sci., 14, 434). Tetramerization is believed to occur preferentially between members of each subgroup (Covarrubias et al. (1991) Neuron, 7, 763-773). The domain responsible for this selective association is localized in the N-terminal region and is conserved between members of the same subgroup. This domain is necessary for hetero- but not homomultimeric assembly within a subfamily and prevents co-assembly between subfamilies. Recently, pore-forming subunits with two TMDs were also shown to co-assemble to form heteropolymers (Duprat et al. (1995) Biochem. Biophys. Res. Commun., 212, 657-663. This heteropolymerization seems necessary to give functional GIRKs. IRKs are active as homopolymers but also form heteropolymers.